On 26 jun, 14:35, "amdx" wrote:
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth
F=18 inches Ok so far.
How do you find beamwidth? I think the 35" dimensiom would have a tighter
pattern than the 20 ", but how do you calculate,

Second, The feed antenna, There can be different feeds.
A simple monopole, dipole, biquad, patch, yagi, helix.
How does the feed antenna affect the gain of the dish, if at all?
Yea, it my be difficult to light the yagi or helix, so if you like throw
those out.
Thank You,
Mike

Hello Mike,

In case of uniform illumination, gain is maximum, side lobes are
maximum also and beam width is about 60*lambda/diameter (degrees).

You will certainly not reach this, for several reasons. The beam width
will be more, it can be twice as high (depending on the illuminator).

The maximum gain that you can obtain with a certain aperture antenna
is about 4*pi*A/lambda^2. In reality it will be less:

When you want uniform illumination, you will have "spill over" (part
of the radiated power will not reach the dish). To get most power onto
the dish, you must accept that the edges of the dish do receive less
power flux density.

In addition, the phase center of your illuminator will change versus
off-main beam direction. This causes phase differences so a non-planar
field. Also polarization may change for off-beam directions.

Other things are deviation from optimal parabolic shape, non correctly
positioned illuminator, etc. This will all reduce maximum gain.

Hope this helps a bit.

Best regards,

Wim
PA3DJS
www.tetech.nl